The present invention relates to a rotary-wing aircraft swashplate assembly, and more particularly to a swashplate assembly with a compound bearing having degrees of freedom in axial, radial and pivot directions so as to replace one or both of the stationary and/or rotating scissors.
Control of a rotary-wing aircraft is affected through cyclic and collective pitch control. Blade pitch control of a rotary-wing aircraft main rotor system is typically achieved through a swashplate assembly which transfers the motion of non-rotating servo-driven control members within a stationary field to the rotating members within a rotational field.
The swashplate assembly generally includes two rings connected by a series of bearings with one swashplate ring connected to the airframe/gearbox (stationary field), and the other swashplate ring connected to a rotor hub (rotational field). Apart from rotary motion, the rotationally stationary and rotational swashplate otherwise move as a unitary component. The swashplate assembly also includes a pivoted link device typically referred to as a “scissors” to coordinate rotationally stationary or rotational swashplate motion within the respective field.
A rotating scissor assembly interconnects the rotating swashplate to the rotor hub and a stationary scissor assembly interconnects the airframe/gearbox to the stationary swashplate. The rotational swashplate rotates relative the stationary swashplate.
Each scissor assembly includes an upper link and a lower link each attached together through a bolt and respective journal-thrust bearing. The lower link of the rotating scissor assembly is typically attached to the rotating swashplate by a spherical bearing. A similar arrangement exists for the stationary scissor assembly.
Although operationally effective, one disadvantage of the existing two-link scissor assembly design is the relatively high part count in which one typical scissor assembly may include two links, three bolts, five beatings and other hardware.
Another disadvantage is directly related to the innate geometric inefficiency of the scissor design in transmitting torque to the swashplate. That is, the conventional two-link scissor assembly design locates journal-thrust bearings a distance from the swashplate force. The moment introduced by this offset results in a couple at the journal-thrust bearings, which may be greater in magnitude than the applied swashplate force. To abate these reaction forces the links need to be relatively wide, which may result in a relatively heavy scissor assembly.
A more esoteric disadvantage of the existing two-link design is that by using the journal bearings in a rotating motion, the pressure term and the velocity term in the PV (pressure*velocity) factor become coupled such that PV, which is used to predict the life of the bearing, is unaffected by a change in bearing radius. The reason for this is that although pressure is inversely proportional to the bearing radius, the tangential velocity is linearly proportional to the radius, thereby canceling each other and leaving behind only the length of the bearing as the germane variable. The result is that the lugs of the links have to be relatively wide which then forces the body of the link to increase in cross-section which further exacerbating the weight issue.
Another disadvantage of the conventional two-link scissor design is related to space constraints. In certain rotor head configurations, the space between the rotor hub and the swashplate may limit optimum placement of the rotating scissor assembly (which is as far as possible from the rotor head axis of rotation). This inboard location results in an amplification of the reaction load due to the relatively small moment arm which counteracts the swashplate torque (in some rotary-wing aircraft, an amplification factor of 3 from the pitch control rod loads is necessary). The overall result is that the scissor links and the bolts which attach the scissor assembly also have to be of significant robustness to manage the magnitude of the vibratory load.
Furthermore, conventional two-link scissor assembly designs may require “shimming” to eliminate manufacturing tolerances at assembly. This procedure preloads the bolt such that the journal-thrust bearings remain in contact during operation. Shimming may be relatively time consuming and may increase the possibility of foreign object damage (FOD).
Accordingly, it is desirable to provide an uncomplicated, light weight, anti-torque mechanism for a swashplate assembly having degrees of freedom in axial, radial and pivot directions so as to replace one or both of the stationary and/or rotating scissor assembly.